Vector magnetometry exploiting phase-geometry effects in a double-resonance alignment magnetometer

Double-resonance optically pumped magnetometers are an attractive instrument for unshielded magnetic-field measurements due to their wide dynamic range and high sensitivity. The use of linearly polarized pump light creates alignment in the atomic sample, which evolves in the local static magnetic field, and is driven by a resonant applied field perturbation, modulating the polarization of transmitted light. We demonstrate experimentally that the amplitude and phase of observed first- and second-harmonic components in the transmitted polarization signal contain sufficient information to measure the static-magnetic-field magnitude and orientation. We describe a laboratory system for experimental measurements of these effects and verify a theoretical derivation of the observed signal. We demonstrate vector-field tracking under varying static-field orientations and show that the static-field magnitude and orientation may be observed simultaneously, with an experimentally realized resolution of 1.7 pT and 0.63 mrad in the most sensitive field orientation.